The theory is this. Eerie really is as boring as it appears to be but Milkman!Marshall causes the events to happen to provide his younger self with a fascinating childhood. He hits and kills two children, knowing how they'll affect his younger self; he leaves his milk truck unattended when it's most important and he probably had his hand in other things behind the scenes. Due to the presence of some kind of stable time loop, by being the Milkman and creating the weirdness, the weirdness exists for him to become the Milkman in the first place. This also explains why Marshall seems to be the only one aside from Simon and Dash that realize anything is weird in Eerie. All the weirdness is centralized around him specifically because it's tailor-made for him.
- That doesn't explain Sarah's predicament.
- Also, it is unlikely Marshall will try to murder Devin (who is shown as his good friend) for Melanie who, after all, ends up leaving him. And how do we even begin to explain Bigfoot, Elvis, the conspiring dogs, and general weirdness and of much of Eerie's history and cultural institutions ("Harvest King", "Loyal Order of Corn", "Grungy Bill", etc.)— Clearly the weirdness cannot be the work of a single man! The Milk Truck was probably an in-joke among the creators and nothing more. If there was ever to be an in-series explanation, it would have been far weirder than Marshall the Murdering Milkman!
- It is more likely that Marshall ends up falling in love and perhaps ends up marrying Janet Donner, as Janet was shown to be as resourceful as Marshall himself. Marshall-the-old-man wants to make sure that Marshall-the-boy meets and helps her, fulfilling a predestination loop. This explains why the Milkman/Old-Marshall wanted Marshall to help Janet get home.
Another hint/clue is when a dog from The Retainer says "Eerie today, Indianapolis tomorrow!" Though it is just a figure of speech, it makes sense literally as well.
- Eerie is most likely intended as a generic "small town America" locale, but if we were to guess a location, it would be somewhere along I-69 in the North-eastern part of the state. In "Just Say No Fun", the nurse is shown driving north to Canada and in "Tornado Days", the Tornado is said to be moving "North East into lake Erie" (although it cold be a local lake by the name of "Lake Eerie"). Also, Todd is hitchhiking to Indianapolis, implying to it be a little farther away than a city is to Suburb. Regardless of the geographical information in the episodes, the visual aesthetics and culture of Eerie remind me more of say, Muncie or Lafayette than an Indianapolis suburb (although both cities have a lot more people than Eerie's 16,661). Then, there is also the reference to the Middletown studies (which studied Muncie) when Marshall tells us that his dad was required to move to the "statistically most normal place in the United States" as a part of his job.
- Without identifying where The Track has gone, there's no way Eerie can be Indianapolis. As alluded to above, Eerie is an alternate universe Muncie.
Now look at the series finale again: Marshall finds out his life is a TV show and he's the star, but another guy is about to take his place with the network's blessing. In other words, it's a retool. The last time we saw Dash, he was alone in the world, relatively powerless and searching for answers about his identity. He was scheming and selfish, but he had a conscience and could be counted on to help Marshall out in life-or-death situations. Now he's being treated like a star and has seemingly managed to bamboozle an entire network into making him the main event, something he's willing to murder Marshall to keep. No hint of guilt or even responsibility from Dash. He seems to only know who he is and what he's capable of once he's aware he's a fictional character.
When Marshall runs off, Schaefer makes it clear that if the show doesn't go on, they're cancelled. So Marshall's life and Dash's plan aren't the only things at stake here — the show is over if Dash doesn't win.... And he doesn't. The take-over plan is a bust and Eerie, Indiana is canceled for good. But the show ends on a high note: Marshall is still the star and the show doesn't care if he's doing something "uncool" like going to the movies with his family.
Even though Dash claims to be, well, Dash rather than an actor—he's lying. In actuality, he's the actor Jason Marsden, deluded into thinking that he's Dash (or possible even taking method acting to an extreme, which would be why "Dash" has his own labelled chair on-set). In-universe Dash might not be willing to murder Marshall (if he was, why would he need the pretense of shooting Bigfoot?), but Jason-Dash would.